1. Field of the Invention
The present invention relates to an image pickup lens to be used in an image pickup apparatus such as a digital still camera or a digital video camera. More particularly, it relates to an image pickup lens having four elements in four groups.
2. Description of the Related Art
Image pickup elements such as CCDs (charge coupled devices) and CMOS (complementary metal oxide semiconductors) are being downsized and the number of pixels of such image pickup elements is being increased to improve the image quality due to the recent advancement of the technique of arranging pixels at a micro-pitch.
Then, by turn, the image pickup lens for forming an optical image on a small image pickup element having an increased number of pixels is required to have an enhanced image forming capability and also to be further downsized at reduced cost. However, there arises a problem of tradeoff that it is difficult to enhance the image forming capability of a downsized low cost image pickup lens whereas it is difficult to downsize an image pickup element having a high image forming capability at low cost.
For example, techniques for using a triplet type lens (3 elements in 3 groups) for the image pickup lens and molding at least two of the lenses from synthetic resin have been devised and put to use (see, Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 9-133860).
However, when a triplet type image pickup lens is prepared by using less costly synthetic resin, there arises a problem that it cannot accurately form an image on an image pickup element having a large number of pixels, or more than 3 million pixels to be more specific. When the number of lenses of the image pickup lens is increased simply to improve the image forming capability, the image pickup lens comes to show a large thickness as a whole to make it impossible to meet the request of downsizing.